Rotation Effects in Conservation Agriculture Systems of Southern Africa: Understanding the Benefits.

Conservation agriculture (CA) systems based on minimal soil disturbance, crop residue retention and crop rotations are increasingly promoted in southern Africa. In the past, much emphasis was placed on investigating CA system. However, little is known about crop rotations, apart from their ability to break pest and disease cycles and to increase soil fertility if leguminous crops are involved. On a multi-seasonal long-term trial in Monze, southern Zambia, we closely monitored the effects of conventionally ploughed and CA systems on soil quality, water dynamics and overall crop performance from 2005-2009. The trial includes monocropped maize and different maize rotations with cotton and the green manure cover crop sunnhemp (Crotalaria juncea L.). Throughout the investigated period we discovered increases in organic matter on CA plots especially on rotations involving cotton and sunnhemp. Soil biological activity was higher on CA plots with highest earthworm numbers found in maize seeded after sunnhemp in a three course rotation (maize-cotton-sunnhemp). Water infiltration was higher and available soil moisture was greater on CA plots with maize in rotation compared to conventionally ploughed check plots. Increases in fertility and rainfall use efficiency led to higher yields on CA plots.  Highest yields were found in 2007/2008 on direct seeded maize following sunnhemp in the three course rotation with 56% higher yields compared to monocropped direct seeded maize. A direct seeded maize crop after cotton had 31% higher yields in the same comparison although no legumes were included in the rotation. This confirms that positive effects from rotations apart from pest and disease reductions can be expected and should therefore be strongly encouraged. The profitability of each CA system will however depend on various factors such as available land area to rotate crops, markets and prices for produce, as well as climatic situation and socioeconomic preferences of farmers.